VDF-based fluoroelastomers are one of the classes of fluorocarbon elastomers mainly available on the market. By virtue of their excellent heat-resistance and chemical-resistance characteristics, these polymers are generally used in the production of compounds intended for the manufacture of technical articles such as sealing parts, pipes, oil seals and O-rings in which the leaktightness and the resistance to substances such as mineral oils, hydraulic fluids, combustibles, aliphatic, chlorinated and aromatic hydrocarbons and solvents or chemical agents of diverse nature must be absolutely ensured, for their use in sectors such as the electronics, aeronautic, nuclear, military, automotive, nautical and energy industries.
Given the specific applications and the particular forms of the products, the conversion techniques applied to fluoroelastomers based on vinylidene fluoride (VDF) have mostly relied on compression-moulding or injection-moulding techniques.
However, the increasing use, in particular in the automotive industry, of profiles and tubes based on such materials has recently encouraged the development of extrusion-based techniques for converting them. To convert VDF-based fluoroelastomers via this technology, it is essential for the material to be able to be extruded at high velocity, in order to ensure high production efficiency, and for the extruded product to have low die-swell, so as to maintain the desired size and optimum surface qualities, without defects (absence of shark skin). Needless to say, the intrinsic mechanical properties of the VDF-based fluoroelastomer must also be conserved.
It is known in the prior art that the use of processing aids, for example plasticizers such as sebacates, esters and alcohols derived from fatty acids, or hydrogenated polymers such as low molecular weight polyethylene or organic silicones, makes it possible in the extrusion of VDF-based fluoroelastomers to improve the rate of extrusion of the blend and the surface quality of the extrudate. However, such additives have the drawback of degrading the mechanical properties.
It is also known practice in the prior art to use fluoropolymer fillers in VDF-based fluoroelastomer matrices to improve the mechanical properties and the leaktightness of the products. Documents EP 1 262 517, EP 1 260 550 and EP 1 262 497 disclose fluoroelastomer compositions in which disperse TFE-based fluoropolymer fillers are incorporated into a VDF-based fluoroelastomer matrix with a high Mooney viscosity (between 49 and 52 MU), for the manufacture of O-rings and other products.
Also, document U.S. Pat. No. 6,310,141 discloses fluoroelastomer compositions comprising a VDF-based fluoroelastomer matrix and a fluoropolymer of THV type (tetrafluoroethylene (TFE), hexafluoropropylene (HFP), vinylidene fluoride (VDF)) comprising small amounts of TFE-based repeating units in the chain. However, these THV fluoropolymers do not make it possible to obtain an improvement in the extrudability of the elastomer (die swell, surface qualities), e.g. a shear rate of about 100 sec−1, typical of extrusion processes.
The fluoroelastomer compositions of the prior art are not suitable for conversion by extrusion, since they cannot be processed at high speed and give rise to pronounced die swell of the extrudate and/or defects in the surface quality.